Evolutionary shifts in olfactory sensitivities in fruit flies

Shifts parallel one fly's 'superspecialized' attraction to a single fruit

How do an animal's senses change as it evolves to occupy a new ecological niche? By comparing the olfactory system of the fruit fly Drosophila melanogaster, which feeds from multiple fruit types, with that of its sibling species D. sechellia, a specialist particularly drawn to a single fruit type, researchers have demonstrated how evolution can act on several different levels of a sensory system to create a supersensitive detection system for a specific food source and egg-laying environment.

The findings are reported by researchers Teun Dekker, Irene Ibba, Purayil Siju, Marcus Stensmyr, and Bill S. Hansson of the Swedish University of Agricultural Sciences.

Most insects rely on their olfactory system to detect mates, food, and egg-laying sites. This also holds true for fruit flies in the genera Drosophila. Both the peripheral and the central olfactory systems have been mapped in detail in D. melanogaster, an important model organism for research. However, nine sibling species of D. melanogaster are less well characterized. One of these species, D. sechellia--as its name suggests, it is endemic to the Seychelles islands--relies exclusively on one fruit, called the morinda fruit, for egg-laying. This fruit, which smells of gorgonzola and pineapple, is toxic to, and shunned by, D. melanogaster and other sibling fruit fly species.

In their new work, the researchers show that the main cue used by D. sechellia when locating morinda fruit is methyl hexanoate (MeHex), which possesses a pineapple-like odor. The researchers found that olfactory hairs (sensilla) on the antenna that house receptor neurons specific to the MeHex odor have become three times more numerous, and one hundred times more sensitive, in D. sechellia than in the sibling species D. melanogaster. In parallel, a specific brain area (a so-called olfactory glomerulus) that receives input from the MeHex-specific neurons is significantly increased in size in D. sechellia. The researchers' findings indicate that the relative overexpression of MeHex-specific receptor neurons in D. sechellia compared to D. melanogaster has occurred at the expense of two other types of sensilla.

The work suggests that the evolution of a specialized olfactory system can occur during a limited evolutionary time span, and at several different levels of neural organization. More broadly speaking, the findings contribute to our understanding of the evolution of sensory systems and their adaptation to new conditions and resources.

###

The researchers include Teun Dekker, Irene Ibba, K.P. Siju, Marcus C. Stensmyr, and Bill S. Hansson of the Swedish University of Agricultural Sciences in Alnarp, Sweden. This work was supported by an EU Marie Curie fellowship to T.D. and by a Swedish Research Council (VR) grant to B.S.H.